1. Field of the Invention
The present invention relates to the field of image sensors and imaging systems.
2. Description of the Related Art
An integrated image sensor is used to convert light impinging on the sensor into electrical signals. An image sensor typically includes one or more (e.g., an array of) photoelements such as photodiodes, phototransistors, or other types of photodetectors, where electrical signals are generated via the well-known photoelectric effect. These signals may then be used, for example, to provide information about light intensity, color, or the optical image focused on the sensor. One common type of image sensor is a CMOS image sensor.
FIG. 1 shows a schematic top view of a CMOS image sensor 100 implemented in a single integrated circuit or chip. Sensor 100 comprises a photoclement array 102, a decoding/buffer area 104, and control, processing, and input/output (I/O) circuitry 106. Photoelement array 102 comprises an array of photoelements and associated circuitry such as switches and amplifiers. Each photoclement and its associated circuitry are collectively referred to as a pixel. Image sensors, such as sensor 100, may be used in imaging systems, such as digital cameras.
Testing and manufacturing yield can have a substantial influence on the ultimate cost of a chip. Testing is done to detect circuit defects to prevent customer returns. Ideally, the defects are detected early in the manufacturing process to avoid unnecessary fabrication costs for defective chips. Often, such testing is carried out using high-speed testing systems on the bare die on a wafer before the circuit is packaged.
FIG. 2 shows a schematic block diagram of a typical testing system 200 that can be used for testing image sensors, such as sensor 100 of FIG. 1, prior to; packaging. Testing system 200 comprises a testing platform 202, a controller 204, a power supply 206, and a stimuli generator 208. The device under test (DUT), in this particular case, unpackaged image sensor wafer 100, is mounted on platform 202, which comprises control and support circuitry 210 and an interface 212. Controller 204 is a software-driven device that controls platform 202, supply 206, and generator 208. Generator 208 is a calibrated light source that provides optical input for sensor 100 when instructed to by controller 204. Controller 204 performs a specific test function on sensor 100 using interface 212 and circuitry 210. The overall test procedure may have a sequence of such functions. Controller 204 receives test data generated by sensor 100 through interface 212, analyzes the data, and determines if sensor 100 performed according to the specifications using a set of predetermined criteria stored in the controller""s memory. If the criteria are satisfied, then sensor 100 is marked for later packaging. If the criteria are not satisfied, then the sensor is marked defective and is usually discarded.
A problem with this approach is that testing on the bare die may not reveal all defective sensors. Unlike many other integrated circuits, integrated image sensors, such as sensor 100, due to certain specific characteristics, also have to be tested after final assembly. For example, an image sensor should be free of (1) optical obstructions in the photosensitive area and (2) optical system defects, such as microlens defects. These types of defects can be detected only after the chip has been packaged.
To accommodate this requirement, a test procedure may involve multiple testing stages. For example, in a first testing stage, often referred to as prescrcening, a fast relatively simple test is performed on an unpackaged sensor, for example, using the testing system of FIG. 2. The sensors that fail the prescreening are discarded, while the sensors that pass the prescreening are packaged. In a second testing stage, a more comprehensive test is performed on each packaged device. As with unpackaged sensors, packaged devices that fail the second testing stage are discarded. This second testing stage is often implemented using a testing system functionally similar to testing system 200 of FIG. 2. However, in this case, the testing system is designed to simulate the operation of the packaged sensor in the final product. Consequently, testing systems for the prescreening and the second testing stage may need different equipment. The expense of building multiple testing systems often precludes (1) the use of several testing systems in parallel to speed up testing and/or (2) the use of duplicate testing systems at separate locations. Lastly, there is a need for testing once the sensor has been incorporated into the final system (e.g., a camera). This kind of test is often the most onerous, since the entire final product has to be placed under test, and the hardest to perform, since the sensor will be at that stage connected to the rest of the circuit and the control and diagnostics software might not yet be built into the final system. A system that can self test, detect errors, and correct such errors is not currently available, and would be of great value to many businesses and industries.
The present invention provides an integrated system-on-a-chip (SOC) imaging system with built-in diagnostics. According to one implementation of the present invention, an imaging system can be operated in two operating modes: a normal operating mode and a diagnostic mode. While running in the diagnostic mode, the imaging system can be configured to detect manufacturing defects for identifying defective chips. In certain embodiments, the imaging system can be further configured to compensate for certain types of manufacturing defects. While running in the diagnostic mode, the imaging system (1) identifies pixels that function incorrectly and (2) creates a record of such pixels. In the normal operating mode, the imaging system can use the record to compensate for the missing or incorrect data from these defective pixels during real-time image processing. The present invention simplifies testing of image sensors by providing an SOC image sensor that can be tested one time using a relatively simple testing system as opposed to the relatively complex multi-stage multi-system testing of the prior art. It also increases manufacturing yield by providing compensation for certain types of sensor defects and, therefore, results in lower per-unit manufacturing cost.
According to one embodiment, the present invention is an imaging system comprising an image sensor, a memory, and a processor, wherein the image sensor is configured to generate image signals corresponding to an image of a scene; the memory is configured to store image data corresponding to the image signals; and the processor is configured to control operations of the imaging system in a diagnostic mode and in a normal operating mode, wherein, during the diagnostic mode, the processor analyzes the image data to determine if the image sensor is defective.
According to another embodiment, the present invention is a method for fabricating an imaging system comprising the steps of (a) forming an image sensor configured to generate image signals corresponding to an image of a scene; (b) forming a memory configured to store image data corresponding to the image signals; and (c) forming a processor configured to control operations of the imaging system in a diagnostic mode and in a normal operating mode, wherein, during the diagnostic mode, the processor analyzes the image data to determine if the image sensor is defective.
According to yet another embodiment, the present invention is an imaging system comprising an image sensor, a memory, and a processor, wherein the image sensor is configured to generate image signals corresponding to an image of a scene; the memory is configured to store image data corresponding the image signals; and the processor is configured to control operations of the imaging system in a normal operating mode, wherein, during the normal operating mode, the processor processes the image data to compensate for one or more defective pixels in the image sensor.